This invention relates to changeable pole three phase winding electric machines in general and more particularly to such a machine in which any number of pole-pairs in the ratio p.sub.1 : p.sub.2 = 3m : 2n can be obtained using a simple, symmetrical, low harmonic content winding where p.sub.1 :p.sub.2 is a fractional number with aliquant numerator and denominator.
Pole changeable Machines are known which include a base winding formed by nine base winding branches each having the same number of series connected winding poles with three connecting points for each of the two pole pair numbers. The three connecting points for the pole pair number p.sub.2 = 2n are connected with three respective ends of the base winding branches.
Three phase windings in which the poles can be changed find particular application for changing the speed in asynchronous squirrel cage motors in a step like manner. A motor winding of this type having only six connecting points is described in the journal "Revue Generale d'Electricite", in particular pages 323-329. There a 6/4 pole three phase winding is described, this winding respresenting a special case of what are known as "phase modulated" windings which usually have more than six connection points. The disclosed changeable pole 6/4 pole three phase winding has a first winding part having a triple Y-connection and separate neutral points. This winding part is effective, i.e. carries current, for both pole numbers. A second winding part is connected to the ends of the first winding part which are tied together and carries current only for the six pole operation. In this article no details are given regarding the 8/6 pole three phase windings which are mentioned. For the 6/4 pole winding, the well known rule that the phases of the 4 pole winding cannot be divided, in principle, into three parallel branches of identical design holds true. As a result, this known pole changeable three phase winding generates a rotating field containing strong harmonics with subharmonics [.nu. = 1/2, 5/2, 7/2, etc.] and even-numbered harmonics which have adverse effect on the operating characteristics of the three phase machine during 4 pole operation. These subharmonics and even field harmonics generate, particulary in asynchronous machines with a relatively small air gap, parasitic effects such as, noise, vibrations, harmonic torques or shaft voltages.
In view of this, the need for an improved pole changeable motor having a simple switching arrangement, which is capable of providing a mutually independent adaptation of the effective number of turns of both, basically arbitrary pole number combination within the mentioned pole number ratio, using simple and, as far as possible, identical coil windings for the entire winding and at least obtaining the maximum symmetry possible, and through such measures obtaining complete suppression of even and subharmonic field harmonics is evident.